IR sensors are used to measure temperatures and/or provide images of remote objects in a scene by detecting the infrared radiation emitted from the target object impinging upon a sensing array. Likewise, IR sensors with a single pixel are used to measure the temperature of an object that illuminates the whole field of view of that sensor.
Ideally the sensing array should output a uniform response when it views an infrared source that produces a uniform amount of radiation.
A sensing array typically comprises an array (grid) of pixels each individually responsive to infrared radiation. In for example a microbolometer, each pixel consists of a thermally isolated “bridge” of resistive material that is heated by incident radiation. The resistance of the bridge varies with its temperature and this variation in resistance is used to generate an output related to the intensity of incident radiation. Another example of thermal infrared sensing devices are sensors based on thermopiles.
In practice infrared sensor arrays are subject to a large amount of non-uniformity between pixels i.e. when exposed to the same amount of radiation each pixel produces a different response. The raw output from such arrays is dominated by this effect and is not recognizable as an image. As this is the case, infrared cameras known in the art apply a correction to the raw output of the array. A known correction is to generate a table of individual correction factors to be applied to the outputs of each pixel in the image.
Additional problems are encountered if the temperature of the sensing array varies, for example as a result of local hot spots in the sensor or by external heat sources surrounding the sensor, as the appropriate correction factors also vary with temperature. When the temperature of the sensor array varies, the latter creates inevitable linear and non-linear thermal gradients over the sensor.
A thermal gradient over the pixels results in a different pixel temperature for each of the pixels. The thermal infrared sensor with its pixels is situated in a certain environment. That environment has a temperature and it could also be that the environment is closed by a certain boundary (e.g. a cap on top of the sensor or another object at a certain temperature). Because of the thermal heat transfer between the environment or cap or other object towards the pixels, the pixels could be heated up differently because of the fact that their pixel temperatures are different or because of the fact that the thermal resistance from the environment to the pixels is different.
One way in which this problem is dealt with is to characterize the array performance at one or more temperatures and then to use dedicated sensors provided on the array, typically blind pixels, thermistors or similar, to measure the current array temperature. A temperature dependent interpolation can then be carried out to estimate a suitable adjustment to the correction factors. However, due to the extreme sensitivity of the correction factors to array temperature, it is difficult to apply this technique sufficiently accurately to provide an accurate temperature measurement.
There is still room for improvement or alternatives.